1. Field of the Invention
This invention relates to a method of producing a bonded wafer by bonding a wafer for an active layer ion-implanted with a light element such as hydrogen or helium to a given depth position onto a wafer for a supporting substrate, and thereafter exfoliating the wafer for the active layer at the ion-implanted portion of the given depth position through a heat treatment to leave an active layer on an outer peripheral portion of the wafer for the supporting substrate and then polishing a terrace portion of the active layer formed on the outer peripheral portion of the wafer for the supporting substrate due to sagging of an outer peripheral portion of the wafer for the active layer during the exfoliation.
2. Description of Related Art
As the method of producing the bonded wafer, there is a so-called smart cut method wherein a wafer for an active layer ion-implanted with a light element such as hydrogen or helium to a given depth position is bonded onto a wafer for a supporting substrate and thereafter subjected to a heat treatment to exfoliate the wafer for the active layer at the ion-implanted portion of the given depth position. In the smart cut method, the exfoliated portion of the wafer for the active layer after the bonding can be recycled as a wafer, which is different from the conventional bonding technique. By such a recycling it is made possible to use one wafer in the bonded wafer plural times, which leads the way to reduce the material cost. Also, the bonded wafer produced by the smart cut method is excellent in the thickness uniformity, so that this method is noticed as a promising method.
In the smart cut method, however, when the wafer for the active layer is exfoliated through the heat treatment, a terrace portion of the active layer is inevitably formed on the outer peripheral portion of the wafer for the supporting substrate due to the sagging of the outer peripheral portion of the wafer for the active layer during the exfoliation and also a part of the wafer for the active layer is firmly fixed onto the terrace portion to form island-shaped projections called as so-called SOI islands. If the SOI islands remain on the terrace portion, there is a fear that they render into particles during device processing to easily cause a pattern defect badly exerting on the characteristics or performances of the device. Therefore, in order to improve the quality of the bonded wafer, it is necessary that the SOI islands are removed by polishing the terrace portion to render the surface of the terrace portion into a clean state having no SOI island or the like.
As a first method of removing the SOI islands retained on the terrace portion is mentioned a method of suppressing the occurrence of SOI islands. For example, JP-A-2003-347526 discloses a method wherein the occurrence of SOI islands is reduced by changing an exfoliation annealing condition so that the temperature is raised from an annealing start temperature at a moderate gradient up to a dislocation temperature and thereafter the heating is conducted at a rapid gradient.
As a second method of removing the SOI islands retained on the terrace portion is mentioned a method of suppressing the occurrence of the terrace portion. For example, JP-A-2000-243942 discloses a method wherein a supporting substrate, an oxide film and an active layer are stepwise piled one upon the other to make an adhesion area between the outer peripheral portion of the active layer and the supporting substrate to thereby decrease the occurrence of SOI islands.
As a third method of removing the SOI islands retained on the terrace portion is mentioned a method of removing the terrace portion. For example, JP-A-2001-144274 discloses a method wherein the active layer is exfoliated through a heat treatment and thereafter an end portion of the bonded wafer is subjected to a beveling treatment to remove the terrace portion.
In the method of JP-A-2003-347526, however, it is impossible to completely remove the SOI islands. Also, the smoothness on the outer peripheral face of the active layer, which is an existing range of SOI islands on the outer peripheral portion of the active layer, is as large as not less than 20 μm, so that there is a fear that these islands are exfoliated to form particles in the subsequent device processing.
In the method of JP-A-2000-243942, since the end portion of the bonded wafer is stepwise, the end portion of the wafer for the supporting substrate becomes wasted. In the method of JP-A-2001-144274, since the end portion of the bonded wafer is removed by the beveling treatment, the thin active layer has a fear of causing contamination or damage through the beveling treatment, and hence a part of the effective active layer is cut out. As a result, there is a problem that the effective area of the wafer usable as a semiconductor substrate is made small.